As is commonly known in the conventional technology, in diesel engines used as automobile engines or the like, fuel injection control can be performed to adjust the timing and amount of fuel injection from a fuel injection valve (hereinafter also referred to as an “injector”) in accordance with the engine speed, the amount of accelerator operation, the coolant temperature, the intake air temperature, and the like (see, for example, PTL 1).
Combustion in a diesel engine is accomplished by pre-mixed combustion and diffusion combustion. When fuel injection from a fuel injection valve starts, first, a combustible air-fuel mixture is produced by vaporization and diffusion of fuel (ignition delay period). The combustible air-fuel mixture then self-ignites approximately simultaneously at several places in a combustion chamber, and combustion rapidly progresses (pre-mixed combustion). Furthermore, fuel continues to be injected into the combustion chamber, so that combustion takes place continuously (diffusion combustion). After that, there is non-combusted fuel even after completion of fuel injection, so heat continues to be produced for a while (afterburning period).
In addition, in a diesel engine, as the ignition delay period becomes longer, or as the vaporization of fuel in the ignition delay period becomes more intense, the flame propagation speed after ignition increases. When the flame propagation speed is high, too much fuel combusts at a time, and the pressure inside the cylinder increases sharply, generating vibration or noise. This phenomenon is called “diesel knocking”, and often occurs particularly when operating at low loads. Also, in such a situation, the amount of production of nitrogen oxide (hereinafter referred to as “NOx”) increases along with a sharp increase in combustion temperature, deteriorating exhaust emissions.
In order to prevent such diesel knocking and reduce the amount of NOx production, various fuel injection control apparatuses have been developed. For example, intermittent injection is commonly performed in which fuel injection from a fuel injection valve is performed intermittently by splitting it into a plurality of injections.
For example, according to PTL 2, the injection timing of a pilot injection, in which fuel is injected prior to a main injection, is set to be earlier by an amount equal to a delay period from the injection timing of pilot injection to the actual ignition timing. As a result, the ignition timing of the fuel injected in the pilot injection can be set substantially the same as the main injection period, thereby reducing the amount of PM (particulate matter) and HC (hydrocarbons) emissions.